Oxygen separation



Mardi 7 1950 H. J. oGoRzALY 2,500,136

OXYGEN SEPARATION Filed June 18, 194e fa-NPY d' OGoeznL Y,

rroP/vm/ Patented Mar. 7, 1950l OXYGEN SEPARATION Henry J. 4Ogorzaly, Summit, N. J.,l asslgnor to Standard Oil Development Company, a corporation of Delaware Application June 1s, 194s, serial No. 617,501 4 claims.` (ci. ca -175.5)

The present invention is concerned with an improved process for the manufacture of gaseous oxygen and nitrogen from air. It is more particularly directed to .a process for the removal of yacetylene and other undesirable constituents which tend to accumulate in the oxygen reboiler in a two tower fractionatlng system. In accordance with 'the present invention acetylene is removed from the fractionating system by withdrawing from the bottom of the separating tower an acetylene-containing oxygen-rich liquid stream, recirculating this stream through illtering means to remove the acetylene and then returning the acetylene free liquid stream to the system, or vaporizing the acetylene-containing liquid stream yby heating and adding the vaporized gases' to the product oxygen stream.

It is well known in the art to manufacture oxygen and nitrogen from liquid air by fractionating the same. In these processes the air is cooled to liquefaction temperatures by various means and processes and is introduced to various fractionating equipment or means. A conventional method employs two fractionating towers, one a high pressure tower, and the other a low pressure tower. In an air separation process of high efllciency the air is chilled to about -275 F. and introduced into the bottom of a. high pressure tower. 'I'he pressure maintained in this tower may be in the range from about 4.5 to 5.0 atmospheres gauge. This high pressure tower comprises suitable fractionating. plates. Substantially pure nitrogen is removed las an overhead vapor stream from the high pressure tower while the bottom stream comprises an oxygenrich liquid stream containing from about 40 to 50% of oxygen. This oxygen-rich liquid is introduced at an intermediate point into a low pressure tower, which is operated at a pressure of a few pounds per square inch above atmospheric pressure. The low pressure tower likewise contains fractionation means,generally bell cap trays. A vapor stream comprising substantially 100% pure nitrogen is withdrawn overhead from the low pressure tower and a substantially pure oxygen stream is withdrawn as a vapor from a point near the bottom of the low pressure tower. f

The heat required in the low pressure tower to vaporize the liquid feed to the column and also to vaporize the liquid reflux required for eiective separation of the nitrogen and oxygen in this tower, is provided by condensing nitrogen vapor from the top of the high pressure tower. Due to the difference in pressure between the two fractionating columns, the normal temperature differential is reversed and the high pressure nitrogen condenses at a higher temperature than that required to boil the low pressure oxygen. Accordingly, a combination condenserreboiler is generally employed, in which high pressure nitrogen vapors are condensed in tubes immersed in a pool of boiling liquid oxygen in the bottom of the low pressure tower. A portion of the condensed nitrogen is returned to the high pressure tower as liquid reflux, where it provides the liquid wash required for effective fractionation and also supplies the cooling required to produce a liquid bottoms from the vapor feed. Another portion o f the condensed nitrogen is introduced into the top of the low pressure tower to provide theliquld nitrogen wash 'required to free the ascending vapors of oxygen.

Impurities such as acetylene or other light hydrocarbons are present to a small degree in the atmosphere and, in addition, are introduced into the air stream by breakdown of lubricating oil used in the air compressors. At the temperatures of liquid air fractionation, they are solidified and exert a very low vapor pressure, so that they are not vaporized adequately into the gasesous products. As a result, they accumulate in the oxygen reboiler, where, in Contact with pure liquid oxygen, the explosive characteristics of such a mixture'constitute a substantial safety hazard. For the purposes of this specification, it will be understood that in the term acetylene, I include as well, al1 other combustible impurities introduced into an air fractionating system.

An oxygen-rich liquid stream comprising from about 40 to 60% of oxygen and containing acetylene and other impurities in solution is withdrawn from the bottom of the high pressure tower and introduced into an intermediate point in the low pressure tower. Temperature and pressure conditions in the low pressure tower are adjusted so as to remove overhead a gaseous stream of substantially pure nitrogen and from a point near the bottom of the tower, a stream of gaseous oxygen substantially free of nitrogen and containing, in general, less acetylene than is introduced with the air feed. In

accordance with the process of my invention, I

am able to remove acetylene from the system at a rate such that it does not accumulate in the bottom of the tower to a dangerous degree.

I withdraw from the bottom of the low pressure tower the oxygen-rich stream, pass this stream through ltering or equivalent means to remove the acetylene, and then return the acetylene-free stream to the tower. The process of my invention may be readily understood by reference to the attached drawing illustrating one modification of the same.

Chilled feed air which may be at a temperature of about 278 F. and may contain a small percentage of liquid is introduced into high pressure tower 2 by means of feed line I. Temperature and pressure conditions are adjusted to remove overhead by means of line 3 a gaseous stream comprising substantially 100% pure nitrogen. -A liquid bottoms stream comprising 40 to 60% oxygen is withdrawn from the bottom of high pressure tower 2 by means of line 4 and introduced at an intermediate point in low pressure tower 5. Temperature and pressure conditions in low pressure tower 5 are adjusted to remove overhead by means of line 6, a gaseous ,stream comprising substantially 100 pure nitrogen. A stream com- Y prising essentially nitrogen-free oxygen is removed from low pressure tower 5 by means of line 1. The heat supply to low pressure tower 5 is maintained by means of condensing the nitrogen product from high pressure tower 2 which is introduccd into boiler 8, within low pressure tower 5, by means of line 9. This nitrogen is withdrawn by means of line I and a portion is introduced into the top of low pressure tower for reflux, while the remainder is returned through line to the top of high pressure tower 2, to serve as reflux in this tower. A small portion of noncondensable permanent gases is vented from boiler 8 through line 2| in order to maintain efciency of heat transfer.

In accordance with the process of my invention, I withdraw from the bottom of low pressure tower 5 by means of line I I a stream. This stream contains acetylenes which tend to increase in concentration in the bottom of the low pressure tower. By means of pump 22 I circulate this stream by means of line 23 through the reboiler section of low pressure tower 5. A small side stream is passed alternately through filtering means I2 and I3 in order to remove from the stream solid particles of acetylene. While I am passing the stream through one filtering means or its equivalent, I am removing from the other filtering means the acetylene particles. The acetylene free stream is returned to low pressure tower 5 by means of line I4,

A preferred technique of practicing my invention is to continuously withdraw a small portion of liquid oxygen from the circulating stream 23, and to inject this small liquid stream into the gaseous oxygen product stream in line 1 at a suflicientlv high temperature level to ensure vaporizaton of the acetylenes present in the liquid oxygen. This is accomplished by means of line 50. This method has the advantage that the continuous removal of acetylene may be effected while the acetylene in the reboiler is still sufficiently dilute so that it remains in solution in the liquid oxygen. v With the filtration system, the concentration must be allowed to build up to the sature tion point, which represents an increase in the explosion hazard within the reboiler. The temperature level of the gaseous oxygen product stream may he raised by heat exchange to the level necessary to vaporize the small portion of liquid oxygen and the acetylene contained therein. preferably by abstracting heat from the incoming air stream, as shown in exchanger 60.

The process of my invention may be adapted to any fractionating means. It may be applied under varying temperature and pressure conditions. My invention generally covers removing a liquid stream from the bottom of the low pressure tower, treating it in a manner to remove the acetylene and then returning the same to the low pressure tower, or vaporizing at least a portion of the liquid stream under conditions where the acetylenes are also transferred into the vapor phase, and removing the acetylenes in admixture with gaseous oxygen product.

The process of my invention is not to be limited by any theory as to mode of operation but only in and by the following claims in which it is desired to claim all novelty insofar as the prior art permits.

I claim:

1. In a process for the production of nitrogen and oxygen by the successive fractionation of liquid air in high pressure and in low pressure zones, the improvement which comprises introducing the chilled air into the high pressure zone, removing overhead a substantially pure nitrogen stream, withdrawing from the bottom of said high pressure zone a liquid stream rich in oxygen, introducing this stream into an intermediate point of a low pressure zone, removing overhead from said low pressure zone a gaseous stream comprising substantially pure nitrogen, removing from the bottom of said low pressure zone a liquid stream containing acetylene, removing acetylene from said last-mentioned liquid stream, passing the thus purified stream to said low pressure zone and removing from said low pressure zone a gaseous stream comprising oxygen substantially free of nitrogen.

2. The process specified in claim 1 in which acetylene is removed from the liquid stream containing the acetylene by passing the said stream through filtering means to remove solid acetylene particles.

3. Process in accordance with claim 1 in which the pressure on the high pressure zone is in the range from about 4 to 6 atmospheres gauge and in which the pressure in the low pressure tower is in the range from about 1 to 2 atmospheres gauge.

4. Improved process for the production of oxygen and nitrogen from air which comprises, introducing a chilled air stream into a high pressure zone maintained at a pressure in the range from about 4 to 6 atmospheres and maintained at a temperature below about 260 F., removing overhead from said high pressure tower a gaseous stream comprising substantially 100% pure nitrogen, removing as a bottoms stream from said high pressure zone a liquid stream comprising from about 40% to 60% oxygen, introducing last-mentioned stream into an intermediate point of a low pressure tower maintained at a pressure in the range from about 1 to 2 atmospheres. removing overhead from said low pressure zone a gaseous stream comprising substantially 100% pure nitrogen, removing as an intermediate stream a gaseous stream comprising essentially nitrogen-free oxygen, removing as a bottoms stream from said low pressure zone a liquid stream containing acetylene and introducing said liquid stream into the intermediate stream comprising essentially nitrogen-free oxygen under conditions to substantially completel vaporize the acetylene.

HENRY J. OGORZALY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,891,125 Gessel Dec. 13, 1932 2,256,421 Borchardt Sept. 16, 1941 2,408,710 Van Nuys Oct. 1, 1946 

